The present invention relates to a brake proportioning valve of a vehicle, subject to the load of the vehicle.
Such proportioning valves are well known in the art, and their functioning will not be described in detail here.
The document DE-A-3,343,614 describes, for example, a proportioning valve according to the precharacterizing clause of the main claim. This proportioning valve is of the type comprising a first piston sliding in a stepped bore made in a body, and carrying a normally open shutter arranged between a source of fluid under pressure and the associated brake circuit, the first piston being moveable counter to a spring under the effect of the pressure of the fluid which tends to close the shutter, a sliding rod normally bearing between a pusher, the position of which is a function of the load of the vehicle, and the shutter.
Such a proportioning valve has a considerable defect which can become serious in some circumstances. In fact, it is known that such a proportioning valve serves for modulating the pressure of the brake fluid in the rear wheels as a function of the pressure of the source under the load of the vehicle, in order to prevent a locking of the rear wheels during braking. Now during a braking operation, the rear wheels are, of course, relieved of stress.
This is because a load transfer towards the front of the vehicle takes place. Since the cut-off point of the proportioning valve is determined as a result of the setting of the proportioning valve at the factory, and since this setting depends on a chosen compromise taking a particular load transfer into account or not, the braking of the vehicle is therefore at its most efficient only in some highly specific instances.
In fact, if the setting is provided for a medium load transfer, if there is rapid braking in which the load transfer is greater than the medium load transfer provided, because of the response time inherent in the proportioning valve the latter "sees" a heavier load than that which actually exists. The rear wheels are thus braked excessively in relation to what they can support, and this can result in a locking of the rear wheels. This perverse effect is amplified with the amount of braking required and therefore runs completely counter to safety.
If the setting is provided for a maximum load transfer, the opposite occurs, and the rear wheels are not braked sufficiently in relation to what they can support (except during rapid braking). The front wheels then ensure too high a proportion of the braking of the vehicle, thereby making it necessary to oversize the corresponding brakes and possibly giving rise to a locking of the front wheels in the event of poor adhesion.